Internet Protocol Multimedia Subsystems (“IMS”) is an architecture framework for delivering Internet Protocol (“IP”) multimedia to mobile users, such as users of mobile devices. An IMS core network (“IMS core”) permits wireless or wireline mobile devices to access IP-based multimedia, messaging, and voice applications and services during an IMS session. For example, an IMS core may facilitate a session that is a voice call conducted via voice-over-IP (“VoIP”). A mobile service provider, such as a mobile communication and data service provider, may operate or utilize an IMS core in order to provision multimedia services to mobile subscribers across a geographic service region, such as an entire country. A mobile service provider may wish to bill or charge a subscriber for an IMS session using service rates that depend in part on the location of the subscriber during the session.
A mobile service provider may integrate an IMS core with an existing or “legacy” mobile communication infrastructure configured to deliver voice, messaging, and data services to mobile devices. As an example, a mobile service provider may integrate an IMS core with an existing Global System for Mobile Communications (“GSM”)/General Packet Radio Service (“GPRS”) core network that provides voice, messaging, and/or data services to mobile subscribers. The legacy communication infrastructure may implement pre-existing, location-based billing methods. The service provider may, for example, bill a subscriber for a voice call or data transfer switched over the legacy infrastructure using service rates that depend in part on the location of a mobile device. These pre-existing methods may determine the location of a mobile device by evaluating a Cell Global Identification (“CGI”) assigned to a cell site within an existing radio network. Such methods of billing are inadequate for traffic over an integrated IMS core, however, because the legacy methods of location-based billing are not applicable to IP-based traffic.